The present invention relates to rotating disk data storage devices, and in particular, to servo systems used for positioning moveable transducer heads with respect to tracks of data stored on the surface of data disks.
The latter half of the twentieth century has been witness to a phenomenon known as the information revolution. While the information revolution is a historical development broader in scope than any one event or machine, no single device has come to represent the information revolution more than the digital electronic computer. The development of computer systems has surely been a revolution. Each year, computer systems grow faster, store more data, and provide more applications to their users.
The extensive data storage needs of modem computer systems require large capacity mass data storage devices. While various data storage technologies are available, the rotating magnetic rigid disk drive has become by far the most ubiquitous. Such a disk drive data storage device is an extremely complex piece of machinery, containing precision mechanical parts, ultra-smooth disk surfaces, high-density magnetically encoded data, and sophisticated electronics for encoding/decoding data, and controlling drive operation. Each disk drive is therefore a miniature world unto itself, containing multiple systems and subsystem, each one of which is needed for proper drive operation. Despite this complexity, rotating magnetic disk drives have a proven record of capacity, performance and cost which make them the storage device of choice for a large variety of applications.
A disk drive typically contains data recorded in concentric tracks on the surface of one or more rotating flat disks. A moveable actuator positions a transducer adjacent a desired track in order to read from or write to the track. Specially recorded servo fields or servo sectors located at angularly spaced intervals on the disk surface are used to identify and locate tracks of data for positioning the actuator. The servo sector conventionally includes a write recovery field which signals the beginning of the servo pattern, a track field which identifies a track number, usually as a Gray code encoding, and a position error field which is used to generate a position error signal (PES) indicating position of the transducer with respect to track centerline. The transducer periodically reads the servo sectors as the disk rotates to determine its current radial position, and a servo feedback system uses this information to accurately position the actuator to minimize the PES or follow a seek profile.
In order to keep up with advances in other areas of digital data processing technology, disk drive storage devices must make continual improvements in performance, reliability and data capacity. The need for greater data capacity drives a corresponding quest for higher areal density, i.e., the amount of data that can be stored per unit area of disk surface. In recent years there have been enormous increases in areal density of disk drives, and there is every indication that future data storage demand will require still further increases in areal density.
Areal density is approximately related to the product of track density (i.e., the number of concentric tracks per linear unit, measured in a radial direction) and the linear bit density (i.e., the number of bits of data in a track per linear unit, measured in a circumferential direction). Historically, increases in track density have not kept up with increases in linear bit density. One of the main reasons for this is noise in the PES. Other things being equal, the strength of a PES is roughly proportional to transducer head width, which is closely correlated to track width. However, as track width decreases, the standard deviation of PES noise does not scale proportionately, so that the signal-to-noise ratio decreases for decreasing track width.
A need exists for increased track densities in disk data storage devices, and in particular, for improved servo techniques which will increase the signal-to-noise ratio of PES signals.
In accordance with the present invention, a recording disk contains at least two types of servo field, including a first type having a relatively larger track field identifying the track number, and a second type having an abbreviated track field identifying a track within a range of tracks, the range being less than the full range of tracks on the disk surface. Preferably, both track fields are Gray code encodings, the first type of track field contains sufficient Gray code bits to identify the full track number, every Nth servo field contains a track field of the first type, and all servo fields in between contain track fields of the second type, N being at least 2. As a result, the disk area required for encoding a track identifier is reduced.
In the preferred embodiment, the range of tracks of the abbreviated Gray code M is equal to N, the interval of occurrence of a track field of the first type. Preferably, N=M=16, although other numbers could be used.
In the preferred embodiment, the identity of a track during a track seek operation is determined by iteratively assuming a track identity and estimating acceleration of the transducer based on the assumed track identity, until a track identity is found for which estimated acceleration meets some pre-determined threshold. This technique is the subject of above mentioned related patent application Ser. No. 09/930,596 by Ottesen and Smith, entitled xe2x80x9cMethod and Apparatus for Determining Track Identity From Abbreviated Track Identifying Data in a Disk Drive Data Storage Devicexe2x80x9d, although other techniques could alternatively be used.
In the preferred embodiment, the additional disk area made available by Gray code reduction is used to increase the position error field of each servo sector, thereby improving the signal-to-noise ratio of the PES without any change in net data capacity. Alternatively, part or all of the additional disk area made available by Gray code reduction may be used to store additional data. In any case, Gray code reduction provides additional area which may be used is some beneficial manner to either increase the net amount of data stored in a given disk surface, or to reduce the number of errors due to improved PES signal strength, or some combination thereof.
The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: